Friday, August 17, 2012

Trying out the Raspberry Pi

I am a happy owner of a Raspberry Pi - a tiny (and really cheap) computer, capable of running Linux. Now I too am a part of the Raspberry revolution. I am just starting to try it out, but I already got our platform game running on it! More on the game will follow in a later post, first some notes on the Pi itself.

I decided to try out the Raspbian flavor of Linux on the Pi. I chose this for the hardware floating point support, and since the Raspberry Pi foundation recommends it.

I currently don't have an USB keyboard, so I hoped to access the Pi over the network with ssh. Luckily, Raspbian is set up with an ssh server running. The only things required is to make sure that the Pi gets an IP address when it boots, and then find out which IP it got so that you can ssh to it.

I connected the Pi to a laptop running ubuntu, and tried to use the network manager on the laptop to give the Pi an IP address with DHCP when it boots. Sadly, this did not succeed. Apparently the laptop Ethernet interface stays "down" as long as nothing is connected to it (the Pi is off), and the Pi got no IP address when it booted.
Fortunately it was easy setting up a DHCP server on the laptop manually:
1) Turn the network manager off, so it does not get in the way
2) sudo ifconfig eth0 192.168.0.2 #enable eth0, give it an IP address
3) sudo udhcpd -f
4) boot the Pi, wait a while.
5) udhcpd prints out the IP address it offers
6) ssh to the address. User name pi, password raspberry.



If you have the Pi connected to a TV or monitor, you will see the Pi printing some messages while it boots, and then launch the raspi-config program.  Without a keyboard raspi-config is not usable, but you can still access the Pi with ssh. Raspi-config seems to start automatically at each boot, until you have completed it once.  It can be run from a terminal at any time.  If one wants the Pi connected to the internet, some more networking is needed than what I describe above. A router would probably be the most convenient solution. I have had success with Windows "internet connection sharing".

EDIT: When trying to share an internet connection from an Ubuntu laptop to the Pi, this bug bit me. After the suggested work-around, sudo killall dnsmasq on the laptop, I could boot the Pi and it got an IP address. Then the next problem appears: how to find the IP address of the Pi? The Pi prints it on the TV or screen during boot. If no screen is connected one can use the nmap tool:
nmap -sP 10.42.0.1-254
this IP range being the one that Network Manager seems to offer. -sP tells nmap to ping all the hosts in the given range.

I have a PAL TV connected to the Pi. PAL needs a setting in the configuration file config.txt, located on the boot partition (the small, FAT one) on the SD card. Either edit this file on the card and uncomment the line
sdtv_mode=2
or use raspi-config on the Pi.



In this discussion and here I found interesting observations about which SD cards work well with the Pi. Apparently the fancy and expensive cards, called Class 10, are not the best since they are highly optimized for sequential reads and writes. This is good for photography but not for running Linux of the card. For this application, the speed of short random reads and writes is much more important, and these speeds may well be much better on cheaper cards of lower class. I got an 8 Gb Class 4 Sandisk SDHC card from the local Media Markt for 8 €. It seems to work well, but I have no serious benchmarks yet.

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